No, he's never actually seen. Which I think, as David implies, would be a bit of a let-down. With only hearing of him by telephone (and I think a couple of letters), there's always that kind of suspension of disbelief (is that what I mean?) that Hyacinth hasn't realised that he's (almost certainly, anyway) gay. If he appeared, he'd have to adhere to all of the characterisation that's been set out. Ie very stereotypically gay; makes his own curtains, silk pyjamas, tapestry, and all the rest. There'd be no way we could realistically be asked to believe that Hyacinth still sees him as no different to any other male, but keeping him in touch only by phone sustains the possibility that she really is just plain oblivious.
