VoIP: Update strings on Call tiles

[gaelledel]

We need to update the strings on the call tiles
https://www.figma.com/file/V6m2z0oAtUV1l8MdyIrAep/VoIP?node-id=2863%3A45199

Please let me know if we need any other string for call tiles. I can give you the copy for it (e.g: unsure what was discussed/agreed for this issue here matrix-org/matrix-react-sdk#6471)