Unable to use VoIP on router with EE sim

[J55C]

I have a ZTE MC888 router and was using a smarty sim with a VoIP landline plugged into the in built router phone port  and another VoIP landline on a Grandstream WP825 WiFi phone. All services work very well with incoming and  outgoing calls no problem. 
i have a EE pay monthly sim only and yesterday ordered an additional  30 day essential max sim to add to my account  to use with the zte router.

 Really impressed that ordering the sim yesterday it arrived this morning. 😊

 unfortunately problems then started. The router accepted the sim and was giving amazing speeds all devices connected and the VoIP phones but then they kept deregistering or when they did register briefly they only had one way audio . No amount of restarting /resetting made any difference. Inserting the Smarty sim again and the VoIP was all working again. Retried the EE sim and all internet connections fine but no VoIP. It seems that for some reason the router doesn’t like EE for VoIP or EE are blocking the codecs or audio packets to allow VoIP to work 😞 The router is up to date with firmware and also tried an o2 sim in the router worked with VoIP but trying the EE sim again resulted in perfect internet connections to other devices but still one way VoIP or deregistering the phones altogether. Reluctantly I’ve had to cancel the new sim and revert back to Smarty sim. After 1 day of the additional sim I’ll have to remove it from my account. It’s very disappointing that EE sims don’t appear to allow VoIP traffic on routers, but there’s no point in paying for an extra sim if I can’t use it as intended for all services that my router allows. Is this a common issue with EE sims as I would like to maybe try to add an EE sim again at a later date but if it is a common problem then I’ll with have to stay with Smarty for now . With landlines moving to VoIP anyway I thought all networks would allow VoIP accounts to work on different routers 

[qwertyuiopasdfg]

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Thank you for the information, J55C. I called EE, who suggested I ask this question on their forum, as they did not know what VoLP is. I searched before asking and saw your observations, which answered all of my questions. Thank you.

I can add the following: a person I know lives in a remote area. He has been using a TP-Link SIM-only router with an EE data SIM for the past five years, and he can watch TV, Netflix, etc.

I also looked in second-hand shops and asked if they had any data SIM-only routers with VoLP. I received a blank expression from one assistant. However, another assistant said, "I use it all the time." I asked which router he uses. He said, "ZTE MC888. Make sure you buy it direct and that it’s not locked to one network."

Lastly, I wonder what happens in grade-listed areas with none to extremely low signal strengths. Grade-listed buildings do not allow drilling holes to pass fiber into the building.