VoIP Phones - zoiper
You can download a part-configured Zoipier from here:
Configuration instructions for Android and iOS
You can purchase Zoiper here: https://www.zoiper.com/en/shop/buy/zoiper5
If asked about MWI, you can select Disable.
Below is a sample configuration on an iPhone.
1. Download App and go to Settings->Accounts & select 'Yes'
2. Click 'Select a provider'
3. Select 'AAISP'
4. Configure the VoIP credentials from your VoIP number's control page.