sù/sutha vs sòfa

[Tha_Mi_Sgìth]

I've been confused about how zoo is used: sù/sutha.
Colin Mark has "sutha" as feminine, but Am Faclair Beag has both "sù" and "sutha" as masculine.
So I'd expect it to decline as: an sù, anns an t-sù, geata an t-sù (and the same for sutha).
But I've seen "Rathan an Sutha" (BBC Alba), "dhan sù" (in a song), etc. The prefix t- is not being used.
Google confirms that "anns an t-sù" is essentially unused but "anns an sù" is common.
Even though "sù/sutha" is a loan word, how can it behave like any word beginning with s+vowel?
I note that another loan word "sòfa" behaves grammatically: air an t-sòfa, etc.
What's going on with "sù/sutha"?

[faoileag]

I can only assume it's the pronunciation.

sù is essentially an attempt to render "zoo" in Gaelic spelling, though it's pronounced with a Z, which Gaelic doesn't have. The S in sù is therefore not a typical Gaelic S sound, and doesn't feel natural with spelling / pron. rules that belong to the sound S.

Sofa, however, does have an S sound, so sounds more natural with the t- in the usual places.

It's also worth pointing out that loan words do wander a bit between users, both in gender and in the extent to which they obey the rules.

e.g. I would say I'd heard "air a' fòn" more often than "air an fhòn".

No doubt our linguist friends on here will have more scientific explanations.

[Tha_Mi_Sgìth]

Yes, good point about Z, which is a voiced S sound.

Regarding "fòn", there is also the verb form fòn/fònadh.
What do you think of the following:
* Tha mi a' fònadh gu mo bhràthair
* Fònaidh mi a-màireach = I'll phone tomorrow
* Dh'fhòn mi thuige = I phoned him (do you think this sounds right?)
* Tha mi air a dh'fhònadh mu a dheidhinn = I've phoned about it

[faoileag]

You can certainly use fòn, a' fònadh as a verb, but there is an alternative form that is often used (and my own preferred one), cuir fòn gu. That to me sounds more idiomatic, on the analogy of cuir litir / post-d / fios gu.
It also keeps the word fòn unchanged, so no confusion about meaning through lenition etc, though I think that's not a problem in context.

BTW, that last example should just be the verbal noun " Tha mi air fònadh ...".
If there was an object, then the inversion structure would kick in: Tha mi dìreach air Calum fhònadh./ Tha mi dìreach air fòn a chur gu Calum.
The a dh'fhònadh form would only occur after a verb of motion: Chaidh mi a dh'fhònadh Calum. / Chaidh mi a chur fòn gu C.

[Tha_Mi_Sgìth]

Sgoinneil. 'S toil leam "cuir fòn gu" cuideachd.

Mòran taing airson a' cheartachaidh!