If a VoIP line is on permanent divert to another number (mobile or landline) the line is diverted at the network level and so does not traverse over the broadband line at all - so there will be no issue with call quality for incoming calls as the line is diverted – it simply bounces onto the diverted number.
There will however be call forwarding costs for landlines it would be 1p per minute and for mobiles it would be 3p per minute in the UK.
If the person is making outgoing calls (but if the VoIP line is diverted permanently) they would then need to look at their broadband to make sure that they have sufficient bandwidth and speed to support good quality outgoing calls - we use 80kbps as we use a high definition encoding to ensure the best possible quality.