FWP:
For background see S. R. Faruqi's choices. For more on Ghalib's unpublished verses, see the discussion in {4,8x}.
The first line makes a very Ghalibian complaint: the whole age/world offers insufficient scope for the manifestation of tears. (For another such complaint of narrowness, see the verse cited by Zamin, {141,6}.)
The second line then appears to offer a generalized allegorical parallel or illustration of this complaint: the age/world cannot accommodate tears any more than the (available) desert could withstand or sustain or soak up the sea. (The sense of zamaanah as 'world' is ignored by Platts, but appears in Steingass, and is common in Urdu.) The idea of an 'invitation' includes proper accommodation and hospitality for a guest. And the sense of a 'call' or 'claim' (see the definition above) contains an element of challenge. No matter how exactly we take da((vat , it's clear that the desert would never be able to handle the sea.
That kahaa;N , however, offers an extra dimension of pleasure. It can certainly be taken as a scornful rhetorical question, as in the discussion so far. In the well-known proverb kahaa;N raajaa bhoj kahaa;N ganguu telii , 'Where is Raja Bhoj, where is Gangu the oil-presser?!', the implication is clearly that the two are incommensurable, they can't even be mentioned in the same breath. If anyone would consider 'calling' the sea-- well, in comparison to the sea, 'where is the desert?!'.
But then there's also the amusing literal reading-- where, in fact, is the desert? It seems quite possible that the tears, for which the whole age/world is insufficiently spacious, may already have washed it away. It may thus be doubly imprudent for anyone to 'invite' or 'call' the sea. The desert may be not just inadequate, but already gone. Compare the similar theme of {5,4}, in which the heat of the speaker's thought has, apparently quite casually and inadvertently, destroyed the desert.
If the sea is tears (and/or tears are the sea), the logic is clear: both are salty, inexhaustible, overwhelmingly powerful (see {111,16} for an ominous reminder). But then by the same metaphorical logic, the age/world must be a desert, and/or the desert must be the age/world. The power of implication works excellently here.
Asi:
If I would set myself to weeping, then my tears are to such an extent that they cannot be contained within the age/world. Thus I don't at all form the intention of weeping. If there would be the desert, then we might make an invitation to the sea; when there's no desert at all, then an invitation to the sea is useless.
== Asi, p. 263