Server busy

[Hayeshouse]

Since receiving my upgrade phone I've had issues making calls when in London at work. Everything is fine on my home network. When I try to make a call it periodically comes up "server busy, please try later". Happens in SE1, SE1, SE1, SE6.... various areas. I can still send SMS and use my data. This is very frustrating as I work on an ambulance and may need to summon emergency assistance. It's remedied by switching off and on again which takes time obviously. Please help me solve what the problem is. 

[Jenny]

@Hayeshouse - Please try the steps in our network troubleshooting thread.

If you're still experiencing the issue after completing these steps, post the template with your answers here and we'll get back to you ASAP.