If you are going to keep the existing unifi:client:home:matthewsPhone:uptime Channel as a Number (and maybe add UoM support and switch it to: Number:Time 
and Number:Dimensionless for the rssi)
I would say:
unifi:client:home:matthewsPhone:uptimeDuration as a String Type
but… why keep the Number one? better change the existing Channel type to String and keep the same UID: unifi:client:home:matthewsPhone:uptime
edit: I don’t know if using UoM and keeping using Number:Time will be useful to format in the Item label the seconds into other formats… I am not using UoM so much to say if this is possible.
@vzorglub : Do you know if I have a Number:Time Channel Type with values in seconds and an Item linked to it: Can I use formatting in the label to display hh:mm:ss ?
The state of the Item would be: 23454 (seconds) and I would like to display this in another way… is this possible with UoM? (or the Channel needs to be a DateTime type?)
Relevant for UoM: KNX Binding, Sitemap: Number Item is not send to bus